Dry weather brings possible end to mosquito season

Dry weather over the past weeks may signal the end of mosquito season for the year, and with it a lower threat of the mosquito-borne Zika virus that has made headlines for much of the year.

“It has just been a bad year mosquito-wise. It has been one of the toughest years for many, many years.”

The number of suspected Zika cases has dropped significantly from a high of more than 20 per week at the end of the summer to one or two cases per week now, according to Public Health Surveillance Officer Timothy McLaughlin-Munroe.

Bill Petrie, director of the Mosquito Research and Control Unit, said that in recent years the rainy season has lasted into December. “I’ve been doing this too long to get complacent,” he said. But, he added, if the dry weather continues, that will continue to help reduce the mosquito population around the islands.

“It has just been a bad year mosquito-wise,” he said. “It has been one of the toughest years for many, many years.”

But now, he said, “Things have certainly quieted down.”

Last week the Public Health Department said Cayman had its 20th confirmed case of Zika based on blood tests done by the Caribbean Public Health Authority. Most suspected cases of Zika are not tested by a laboratory.

There are two types of mosquitoes the MRCU focuses on. Swamp mosquitoes feed after dark and, as the name implies, breed in brackish swamps. Less rain means they have a much harder time breeding. However, Mr. Petrie said, the higher tides with last week’s supermoon meant that the water levels in Cayman’s swamps have been higher than normal, allowing the mosquitoes there to breed more.

Mr. Petrie said the swamp mosquitoes do not spread viruses but can be a serious nuisance for anyone living along canals and other areas near mangrove swamp.

The Aedes aegypti mosquitoes are responsible for spreading viruses such as Zika, dengue and chikungunya. Aedes aegypti breed in standing fresh water, such as rain-filled flower pots, clogged gutters and discarded tires. Aedes aegypti tend to feed during the day, mostly in the late afternoon.

“Aedes aegypti are purely dependent on rainwater, or people using their garden hoses,” Mr. Petrie said.

“The incidence of Zika seems to be slowing down in the region,” he said, but more cases could be possible. He warned that with holiday travel coming up, Cayman could see more imported cases.

“Lots of people are going to Florida, which has many more cases than we do,” he said.

In a recent email to public health staff, Dr. Samuel Williams-Rodriguez noted the recent decision by the World Health Organization to end the global health emergency it had issued for Zika. Dr. Williams-Rodriguez wrote, “The numbers of suspected cases continue to decline, and we have not diagnosed any case of Guillain-Barre syndrome (GBS), microcephaly or other neurological disorder associated with Zika virus infection in the Cayman Islands.”

The MRCU’s releases of genetically modified mosquitoes continue, even though the dry season has likely reduced the number of Aedes aegypti breeding in West Bay.

In a recent interview with Oxitec’s Renaud Lacroix, he said the releases of genetically modified male Aedes aegypti will continue in hopes of killing off as much of the population in the sample area in West Bay before the rainy season brings the virus-carrying mosquitoes back next year.

Mr. Lacroix said the releases in West Bay are being compared to a neighboring area, allowing Oxitec to measure the success of the trial.